1. Field of the Invention
This invention relates generally to X-ray tubes and is concerned more particularly with a rotatable anode target having a surface coating of amorphous carbon material.
2. Discussion of the Prior Art
Generally, an X-ray tube includes an evacuated envelope having therein an electron emitting cathode disposed to beam high energy electrons onto an aligned focal spot area of a spaced anode target. The target may comprise a rotatable disc having adjacent its outer periphery an annular focal track made of X-ray emissive material, such as tungsten-rhenium alloy, for example. Thus, electrons beamed from the cathode may be focused onto a focal spot surface area of the focal track to penetrate into the underlying material thereof and generate X-rays which pass in a beam out of the tube. Most of the electron energy incident on the focal spot area is converted into heat energy which may become excessive and damage the surface of the focal track. Consequently, the target disc is rotated at a suitable high angular velocity for moving material of the annular focal track rapidly through the focal spot area aligned with the electron beam.
The focal track may comprise a layer of X-ray emissive material annularly disposed on the disc surface adjacent the electron emitting cathode. Therefore, the substrate disc may be made of a suitable lightweight material, such as molybdenum alloy or graphite, for examples, to reduce the inertia of the target and aid it in attaining a desired high angular velocity in a relatively short time interval. Also, the focal track layer may be provided with an optimum thinness for the underlying material of the disc to function as an efficient heat sink in conducting heat away from the electron bombarded material of the focal track. Thus, heat energy is accumulated in the body of the substrate disc for dissipation to surrounding structure of the tube before becoming sufficiently excessive to cause warping or cracking of the target.
However, the target disc generally is mounted for axial rotation by having a central portion thereof attached to a stem end portion of a magnetic induction rotor supported by bearings in the envelope. The stem end portion usually is provided with a suitable cross-sectional configuration for restricting the flow of heat from the target disc to the rotor in order to prevent damage to the support bearings. Consequently, the heat energy accumulated in the target disc cannot be dissipated efficiently by conduction through adjacent structure, and cannot be dissipated by convection since the tube envelope is evacuated. Therefore the heat stored in the body of the target disc must be dissipated predominantly by radiation to the tube envelope, which may be fluid-cooled.
Attempts have been made in the prior art to improve the radiation of heat from the disc to the envelope by providing the target with a surface coating of heat emissive material. However, these prior art target coating materials generally have proved unsatisfactory for numerous reasons, such as poor adherence under thermo-mechanical and electrostatic stress, poor transfer of heat from the disc to the coating and, inadequate thermal characteristics of the coating material, for examples. As a result of poor adherence or poor thermal characteristics, particles of the coating may break away from the target disc. Also, if the substrate disc is made of graphite, carbon dust may be pulled away therefrom by the electrostatic field established between the anode target and the cathode. Thus, loose particles of the coating may be deposited between highly charged electrodes of the tube, or carbon dust may deposit on dielectric surfaces within the envelope. As a result, high voltage arc-over may occur within the envelope, and may cause catastrophic failure of the tube.
Therefore, it is advantageous and desirable to provide an X-ray tube with a target having a tightly adherring coating of heat emissive material, which also prevents the liberation of loose particles within the envelope.